The present invention is related to silicone compositions. More particularly, the present invention is related to low viscosity, curable polydiorganosiloxane compositions.
Dispensable materials that can cure and give high thermal conductivity are typically used in the electronics industry. Currently, there are two classes of cured articles used as thermally conductive sinks. Sakamoto et al., Japanese Patent No. 05117598, discuss highly filled matrices that are cured to make a pad. The pad can be cut and physically placed in an electronic device. Toya, Japanese Patent No. 02097559, discusses a filled matrix that is dispensed and cured in place. The dispensable approach requires that the material have a viscosity that is low enough such that the material can be forced through an orifice for rapid manufacture of many parts. However, the final cured product must have a sufficiently high thermal conductivity.
There remains a need to find a material that has a sufficiently low viscosity such that it can be rapidly placed on a small device with high power requirements. The high power requirement needs a way to remove more heat. This requirement necessitates a thermally conductive material. Thus, dispensable, curable, and high thermally conductive materials are constantly being sought.
The present invention provides a silicone composition comprising a curable adhesive formulation which comprises
(A) a functionalized polydiorganosiloxane having the general formula:
(R1)3-pR2pSiO[(R1)2SiO]m[R1R2SiO]nSi(R1)3-qR2q 
wherein R2 is independently at each occurrence vinylcyclohexeneoxy, silane, epoxy, glycidoxy, acryloxy, imide, urethane, vinyl, or combinations thereof; R1 is independently at each occurrence a C1-8 alkyl radical, phenyl radical, vinyl radical, or combination thereof; xe2x80x9cpxe2x80x9d is 0 or 1; xe2x80x9cqxe2x80x9d is 0 or 1; xe2x80x9cmxe2x80x9d+xe2x80x9cnxe2x80x9d has a value sufficient to provide a polydiorganosiloxane with an initial viscosity in a range between about 100 centipoise and about 50,000 centipoise at 25xc2x0 C.;
(B) at least one reactive diluant;
(C) at least one cure catalyst; and
(D) at least one thermally conductive filler;
wherein the total silicone composition has a viscosity in a range between about 10,000 centipoise and about 250,000 centipoise at 25xc2x0 C. before cure.
The present invention further provides a method for increasing the thermal conductivity of a silicone composition comprising:
(A) providing at least one functionalized polydiorganosiloxane having the general formula:
(R1)3-pR2pSiO[(R1)2SiO]m[R1R2SiO]nSi(R1)3-qR2q 
wherein R2 is independently at each occurrence vinylcyclohexeneoxy, silane, epoxy, glycidoxy, acryloxy, imide, urethane, vinyl, or combinations thereof; R1 is independently at each occurrence a C1-8 alkyl radical, phenyl radical, vinyl radical, or combination thereof; xe2x80x9cpxe2x80x9d is 0 or 1; xe2x80x9cqxe2x80x9d is 0 or 1; xe2x80x9cmxe2x80x9d+xe2x80x9cnxe2x80x9d has a value sufficient to provide a polydiorganosiloxane with an initial viscosity in a range between about 100 centipoise and about 50,000 centipoise at 25xc2x0 C.;
combining into the polydiorganosiloxane at least one thermally conductive filler in a range between about 60% by weight and about 95% by weight of the total silicone composition;
combining into the polydiorganosiloxane at least one diluant; and
combining into the polydiorganosiloxane at least one cure catalyst wherein the total silicone composition has a viscosity in a range between about 10,000 centipoise and about 250,000 centipoise at 25xc2x0 C. before cure.
In yet a further embodiment of the present invention, there is provided a thermal interface material comprising:
(A) at least one polydiorganosiloxane having the general formula:
(R1)3-pR2pSiO[(R1)2SiO]m[R1R2SiO]nSi(R1)3-qR2q 
wherein R2 is independently at each occurrence vinylcyclohexeneoxy, silane, epoxy, glycidoxy, acryloxy, imide, urethane, vinyl, or combinations thereof; R1 is independently at each occurrence a C1-8 alkyl radical, phenyl radical, vinyl radical, or combination thereof; xe2x80x9cpxe2x80x9d is 0 or 1; xe2x80x9cqxe2x80x9d is 0 or 1; xe2x80x9cmxe2x80x9d+xe2x80x9cnxe2x80x9d has a value sufficient to provide a polydiorganosiloxane with an initial viscosity in a range between about 100 centipoise and about 50,000 centipoise at 25xc2x0 C.;
(B) at least one reactive diluant;
(C) at least one cure catalyst; and
(D) at least one thermally conductive filler;
wherein the thermal interface material provides adhesion to at least one substrate.
It has been found that the use of at least one functionalized polydiorganosiloxane, at least one reactive diluant, at least one cure catalyst, and at least one thermally conductive filler provides a formulation with a low viscosity of the total silicone composition before cure and whose cured parts have a high thermal conductivity. xe2x80x9cHigh thermal conductivityxe2x80x9d as used herein refers to a cured total silicone composition with a thermal conductivity greater than about 1.5 Watts per meter per degree Kelvin (W/mK). xe2x80x9cLow viscosity of the total silicone composition before curexe2x80x9d typically refers to a viscosity of the composition in a range between about 10,000 centipoise and about 250,000 centipoise and preferably, in a range between about 20,000 centipoise and about 100,000 centipoise at 25xc2x0 C. before the silicone composition is cured. xe2x80x9cCuredxe2x80x9d as used herein refers to a total silicone composition with reactive groups wherein in a range between about 50% and about 100% of the reactive groups have reacted.
The functionalized polydiorganosiloxane has the general formula (I),
(R1)3-pR2pSiO[(R1)2SiO]m[R1R2SiO]nSi(R1)3-qR2qxe2x80x83xe2x80x83(I) 
wherein R2 is independently at each occurrence vinylcyclohexeneoxy, silane, epoxy, glycidoxy, acryloxy, imide, urethane, vinyl, or combination thereof; R1 is independently at each occurrence a C1-8 alkyl radical, phenyl radical, vinyl radical, or combination thereof; xe2x80x9cpxe2x80x9d is 0 or 1; xe2x80x9cqxe2x80x9d is 0 or 1; xe2x80x9cmxe2x80x9d+xe2x80x9cnxe2x80x9d has a value sufficient to provide a polydiorganosiloxane with an initial viscosity in a range between about 100 centipoise and about 50,000 centipoise at 25xc2x0 C. and a functional content in a range between about 1% by weight and about 10% by weight of the functionalized polydiorganosiloxane. Radicals represented by R1 are preferably C1-4 alkyl radicals and more preferably, methyl. Typically, the functionalized polydiorganosiloxane is present in a range between about 0.5% by weight and about 5% by weight of the total silicone composition, and more typically in a range between about 1% by weight and about 2% by weight of the total silicone composition.
Additionally, a reactive organic diluant may be added to the silicone composition to decrease the viscosity of the composition. Examples of diluants include, but are not limited to, styrene monomers such as tert-butyl styrene (t-Bu-styrene), (meth)acrylate monomers such as methylmethacrylate and hexanedioldiacrylate, methacryloxy-containing monomers such as methacryloxypropyltrimethoxysilane, epoxy-containing monomers such as biscyclohexaneoxyethylenetetramethylsiloxane, glycidoxy-containing monomers such as glycidoxypropyltrimethoxysilane, hydride-stopped polydimethylsiloxanes, and vinyl ethers. It is to be understood that (meth)acrylate includes both acrylates and methacrylates. Vinyl ethers include mono-, di-, and poly-vinyl ethers containing carbon atoms in a range between about 2 and about 20. The preferred reactive diluants are methacryloxypropyltrimethoxysilane and vinyl ethers. The mixture of the diluant and the functionalized polydiorganosiloxane lowers the viscosity, which allows for higher loading of thermally conductive filler. The amount of thermally conductive filler in the silicone composition is directly proportional to the thermal conductivity. Thus, the higher the amount of thermally conductive filler in the silicone composition, the greater the thermal conductivity of the silicone composition.
The thermally conductive fillers in the present invention include all common thermally conductive solids. Thermally conductive fillers in the present invention include, for example, forms of silver, gold, copper, nickel, platinum group metals, and combinations thereof. Silver is the preferred thermally conductive filler. Examples of thermally conductive silver include, but are not limited to, silver powders, silver flakes, colloidal silver, and combinations thereof. Further examples of thermally conductive fillers include, but are not limited to, aluminum oxide, aluminum nitride, boron nitride, diamond, magnesium oxide, zinc oxide, and zirconium oxide. The filler is present in a range between about 60% by weight and about 95% by weight of the total silicone composition, more typically the filler is present in a range between about 75% by weight and about 85% by weight of the total silicone composition.
Inhibitors such as esters of maleic acid (e.g. diallylmaleate, dimethylmaleate), acetylenic alcohols (e.g., 3,5 dimethyl-1-hexyn-3-ol and 2 methyl-3-butyn-2-ol), amines, and tetravinyltetramethylcyclotetrasiloxane and mixtures thereof can also be employed when used in an effective amount which is typically in a range between about 0.01% by weight and about 1% by weight of the total silicone composition.
Adhesion promoters can also be employed such as trialkoxyorganosilanes (e.g. xcex3-aminopropyltrimethoxysilane, glycidoxypropyltrimethoxysilane, bis(trimethoxysilylpropyl)fumarate) used in an effective amount which is typically in a range between about 0.01% by weight and about 1% by weight of the total silicone composition. The preferred adhesion promoter is bis(trimethoxysilylpropyl)fumarate.
Cure catalysts may also be present in the total silicone composition that accelerates curing of the total silicone composition. Typically, the catalyst is present in a range between about 10 parts per million (ppm) and about 10% by weight of the total silicone composition. Examples of cure catalysts include, but are not limited to, peroxide catalysts such as t-butylperoxybenzoate, onium catalysts such as bisaryliodonium salts (e.g. bis(dodecylphenyliodonium hexafluoroantimonate, (octyloxyphenyl, phenyl)iodonium hexafluoroantimonate, bisaryliodonium tetrakis(pentafluorophenyl)borate), triarylsulphonium salts, and platinum catalysts. Preferably, the catalyst is a bisaryliodonium salt. Curing typically occurs at a temperature in a range between about 50xc2x0 C. and about 175xc2x0 C., more typically in a range between about 100xc2x0 C. and about 150xc2x0 C., at a pressure in a range between about 1 atmosphere (atm) and about 5 tons pressure per square inch, more typically in a range between about 1 atmosphere and about 100 pounds per square inch (psi). In addition, curing may typically occur over a period in a range between about 5 minutes and about 1 hour, and more typically in a range between about 15 minutes and about 45 minutes.
The composition of the present invention may by hand mixed but also can be mixed by standard mixing equipment such as dough mixers, chain can mixers, planetary mixers, and the like.
The blending of the present invention can be performed in batch, continuous, or semi-continuous mode. With a batch mode reaction, for instance, all of the reactant components are combined and reacted until most of the reactants are consumed. In order to proceed, the reaction has to be stopped and additional reactant added. With continuous conditions, the reaction does not have to be stopped in order to add more reactants.
Thermally conductive materials as described in the present invention are dispensable and have utility in devices in electronics such as computers or in any device that generates heat and where the device requires the heat to be efficiently removed. The thermally conductive material is typically used as a thermal interface material that provides adhesion to at least one substrate such as silicon, gallium arsenide (GaAs), copper, nickel, and the like.
In order that those skilled in the art will be better able to practice the invention, the following examples are given by way of illustration and not by way of limitation.